1) Field of the Invention
The present invention relates to a sensor apparatus including an oscillation circuit for outputting an oscillation frequency to an A/D conversion circuit and the A/D conversion circuit for receiving the oscillation frequency from the oscillation circuit to conduct sampling processing on analog data on the basis of the inputted oscillation frequency for producing and outputting digital data.
2) Description of the Related Art
So far, there has been known a sensor apparatus (for example, see Japanese Patent Laid-Open Nos. HEI 11-44585 and HEI 11-64135) made up of an oscillation circuit for outputting an oscillation frequency to an A/D conversion circuit and the A/D conversion circuit for carrying out sampling processing on analog data on the basis of the inputted oscillation frequency from the oscillation circuit to produce and output digital data.
There is a problem which arises with the aforesaid Patent Documents, however, in that, since the oscillation circuit is made to output an oscillation frequency based on a power supply voltage directly applied from a power supply terminal, in response to a variation of the power supply voltage, the oscillation frequency to be outputted from the oscillation circuit varies to cause a variation of the sampling time in the A/D conversion circuit. Concretely, for example, when the power supply voltage varies in an increasing direction, the oscillation frequency to be outputted from the oscillation circuit varies in an increasing direction, while the sampling time in the A/D conversion circuit varies in a shortening direction.
As a result of this, reference information A, temperature information T or sensor information (pressure information) D, constituting the digital data to be produced in the A/D conversion circuit, decreases in proportion to the sampling time and this develops the occurrence of an error non-eliminable by the divisions such as T/A or D/A in the subsequent correction arithmetic operations in a digital circuit, so difficulty is encountered in outputting an accurate sensor signal.